Replace IInterface::asBinder() with a static

so we can do NULL checks again, and update calls to IInterface::asBinder()
to use the new static version.

Change-Id: Ia7b10eb38ca55b72278bfd33d3bf647f338b4e6a
diff --git a/libs/gui/SurfaceComposerClient.cpp b/libs/gui/SurfaceComposerClient.cpp
index 547dc7d..1be7895 100644
--- a/libs/gui/SurfaceComposerClient.cpp
+++ b/libs/gui/SurfaceComposerClient.cpp
@@ -71,7 +71,7 @@
     };
 
     mDeathObserver = new DeathObserver(*const_cast<ComposerService*>(this));
-    mComposerService->asBinder()->linkToDeath(mDeathObserver);
+    IInterface::asBinder(mComposerService)->linkToDeath(mDeathObserver);
 }
 
 /*static*/ sp<ISurfaceComposer> ComposerService::getComposerService() {
@@ -462,14 +462,14 @@
 }
 
 sp<IBinder> SurfaceComposerClient::connection() const {
-    return (mClient != 0) ? mClient->asBinder() : 0;
+    return IInterface::asBinder(mClient);
 }
 
 status_t SurfaceComposerClient::linkToComposerDeath(
         const sp<IBinder::DeathRecipient>& recipient,
         void* cookie, uint32_t flags) {
     sp<ISurfaceComposer> sm(ComposerService::getComposerService());
-    return sm->asBinder()->linkToDeath(recipient, cookie, flags);
+    return IInterface::asBinder(sm)->linkToDeath(recipient, cookie, flags);
 }
 
 void SurfaceComposerClient::dispose() {